One multiple access method in a digital radio communication system is CDMA (Code Division Multiple Access). In a standard established by 3GPP (3rd Generation Partnership Project), a standards body related to mobile radio communication systems, this CDMA method is used and a Downlink Shared Channel (DSCH) used for high-speed data communication in a downlink (a channel from a base station to a communication terminal) is stipulated as one channel shared by a plurality of communication terminals (hereinafter referred to as “shared channel”).
As this DSCH is used by being assigned to each communication terminal on a specified transmission unit basis (for example, on a frame-by-frame basis), its use is anticipated in downlink high-speed packet transmission, etc.
In a CDMA system, there are cases where an adaptive array (hereinafter abbreviated to “AAA”) is used with the object of reducing interference. This adaptive array is a technology whereby a base station is equipped with an array antenna comprising a plurality of antenna elements, and directivity is formed by multiplying a transmit signal by a complex coefficient (hereinafter this complex coefficient is referred to as a “weight”), and performing transmission in line with that directivity.
However, since the above-described DSCH is assigned to communication terminals on a per-predetermined-transmission-unit basis, when an adaptive array is applied to DSCH signal transmission, and DSCH signal transmission is performed with a different directivity formed for each communication terminal, directivity is switched every DSCH signal transmission unit. Also, as the DSCH is a channel used for high-speed communication, the DSCH signal power is extremely high compared with a dedicated channel signal.
Thus, when the DSCH is assigned to a particular communication terminal, interference imposed on a dedicated channel signal from the DSCH signal suddenly increases in communication terminals located in the vicinity of a communication terminal to which the DSCH is assigned, and therefore, as shown in FIG. 1, the increase in dedicated channel signal transmission power due to transmission power control in a downlink (a channel from abase station to a communication terminal) (transmission power control that increases or decreases transmission power at a base station according to a transmission power increase/decrease command from a communication terminal) cannot keep up with the increase in interference power conveyed from the DSCH signal. There is thus a problem of the occurrence of major deterioration of dedicated channel signal reception quality (for example, the reception SIR) in communication terminals located in the vicinity of a communication terminal to which the DSCH is assigned, and obstruction of communication. Moreover, since interference power fluctuates abruptly, there is a problem of loss of CDMA system stability and a decrease in system capacity.